The present invention relates to a curing composition for forming a heat-conductive sheet, a heat-conductive sheet, a process for producing the same, a method of bonding or adhering with the use of the heat-conductive sheet and a heat sink structure.
In accordance with the enhancement of the performance of electrical or electronic equipment, there is a trend toward an increase of the number of electrodes used in semiconductor elements and toward an increase of the power consumption by semiconductor elements. Therefore, it is now of importance to efficiently release heat generated by parts of the electrical or electronic equipment.
In order to efficiently carry out the heat sink from semiconductor packages or semiconductors per se, attempts have been made to furnish them with a heat sink device or to effect a heat sink by means of a wiring board having""semiconductor elements mounted thereon. For example, the heat sink from semiconductor packages has generally been performed by natural convection over the surface of heating element body or forced convection by means of a fan disposed in the unit. This system has, however, a drawback in that, when the calorific value is increased in accordance with the enhancement of the function of semiconductor packages, the heat sink capability becomes unsatisfactory to thereby disenable surely preventing any deterioration of the performance of the semiconductor packages. Further, a system comprising a heat sink pressed against the surface of semiconductor packages so as to increase the heat sink by convection has been provided. In this system, however, the contact area of the mutually pressed surfaces of the semiconductor packages and the heat sink is -reduced because of the occurrence of clearance to thereby cause a problem in the exertion of heat sink effect as designed. Accordingly, for example, in the joining of a heat sink to semiconductor packages, it is performed to interpose, for example, a heat-conductive resin sheet therebetween and to, while adhering the semiconductor packages and the heat sink to each other, effectively attain a heat sink. Further, for example, in the joining of a semiconductor element to a heat spreader to be brought into contact therewith, it is performed to interpose a heat-conductive adhesive therebetween and to, while maintaining the bonding of the semiconductor element and the heat spreader to each other, effect the heat sink from the semiconductor element.
As a resin composition for heat conductivity increase or the like to be interposed between the semiconductor element or semiconductor package and the heat sink, for example, a silicone rubber sheet of a claylike thermocuring bonding type is employed in Japanese Patent Laid-open Publication No. 5(1993)-326916. This silicone rubber sheet has a drawback in that the heat conductivity thereof is not satisfactory from the viewpoint of coping with the increase of power consumption being encountered by semiconductor elements. For increasing the heat conductivity, it is performed to randomly disperse metal particles of high heat conductivity in the resin sheet of, for example, silicone rubber. For attaining a further increase of the heat conductivity, it is being tried to load the resin sheet with metal particles in high dispersion and in high filling ratio. However, even if the resin sheet is loaded with metal particles in high dispersion and in high filling ratio, there remain problems not only such that heat is diffused in random directions to thereby disenable satisfactorily increasing the heat conduction between the semiconductor element and the heat sink but also such that the loading with metal particles in high filling ratio causes the resin sheet to have poor tensile strength and poor elasticity and is further likely to cause the resin moldability to deteriorate.
Therefore, there has been a demand for the development of a heat-conductive sheet which is excellent in the anisotropic heat conduction in the direction of the thickness of the sheet (subject 1).
Japanese Patent Laid-open Publication No. 52(1977)-128465 discloses a process for obtaining an electrically conductive sheet while orientating fibers in the direction of the thickness of the sheet by the action of magnetic field.
On the other hand, when a semiconductor package is assembled in, for example, an electronic equipment product and utilized, it has been experienced that the degree of adherence of interposed resin sheet to the semiconductor element and the heat sink is deteriorated because of the suffering of the semiconductor package from vibration, shock, etc. from outside and because of the long-term irregular heat buildup by the semiconductor element, caused by the use of the electronic equipment product, to thereby render the heat sink unsatisfactory. Therefore, it has been demanded for the resin sheet interposed between the semiconductor element and the heat sink or the like to exhibit high heat conductivity and further to not only simply adhere them to each other but also achieve such a satisfactory bonding that these can resist vibration, etc. from outside.
A method of using a resin sheet in combination with an adhesive or a method wherein a highly adherent liquid epoxy resin or the like is applied to adherend surfaces and cured has been tried for improving the bonding between the semiconductor element and the heat sink or the like. However, the former method has a drawback in that the use of an adhesive causes the heat conductivity to deteriorate. The latter method has a drawback in that accurate coating must be effected on minute semiconductor.elements and, in the use of a two-pack epoxy resin, the addition proportion must be controlled to thereby cause the production process to be complex and lack simplicity.
Furthermore, a method comprising thermally curing, for example, a liquid resin for sheet to a semi-cured state, interposing the semi-cured resin sheet between joining surfaces of the semiconductor element and the heat sink or the like and fully curing the semi-cured resin sheet by heating to thereby bond the semiconductor element and the heat sink or the like to each other has been tried. However, this method has a drawback in that it is difficult to control the curing reaction of, for example, highly adherent epoxy resin so that it is terminated at a semi-cured state, thereby causing readily obtaining such a semi-cured product to be practically infeasible.
Therefore, there has also been a demand for the development of a semi-cured resin sheet which not only exhibits a high heat conductivity but also is excellent in the capability of.bonding a semiconductor element or the like and a heat sink means to each other and further can be readily shaped within a short period of time (subject 2).
On the other hand, apart from the demand of the subject 2, if the heat sink or the like is bonded to the semiconductor element to an inseparable degree, there occurs such a problem that, for example, upon finding of any defect in the semiconductor element, the heat sink or the like cannot be easily detached to thereby cause repairing of the semiconductor element to be difficult.
Therefore, there has further been a demand for the development of a heat-conductive sheet which not only exhibits a high heat conductivity but also adheres a semiconductor element or the like and a heat sink means to each other in a separable condition but with a satisfactory adhesive strength, that is, has a sticky surface (subject 3).
As apparent from the above, it is demanded for the heat-conductive sheet to not only exhibit a high heat conductivity but also have various additional functions such as high bonding capability and adherence permitting detachment. Furthermore, besides these demands, there is a demand for a heat-conductive sheet having excellent insulating capability so as to enable protection against electric shock, depending on the type of heating element, which may be experienced when heat sink is conducted on, for example, a circuit board whose surface is not satisfactorily insulated (subject 4).
Moreover, in the field of electrical equipment and machinery other than semiconductor-related components, for example, with respect to high-voltage rotating machines such as a generator and a motor, the allowable calorific value for high-voltage rotating machines per se tends to increase in conformity with the requirement for an increase of the volume of each unit, a voltage increase therefor, a miniaturization and weight reduction thereof, etc. Thus, as for materials employed therein, especially a heat-conductive sheet interposed between the coil and the iron core of high-voltage rotating machines, a further enhancement of the heat conductivity thereof is now an important task, apart from the above demands for the improvement of heat sink performance with respect to semiconductor-related components. Further, there is also a demand for an enhancement of the heat conductivity of heat sink materials for releasing the heat accumulated in, for example, a magnetic circuit of a speaker unit of an audio equipment. Still further, there is such a problem that a lamp housing and other parts are heated to an extreme degree because of an increase of the output of an UV lamp, etc. to thereby shorten the lamp life and to thereby accelerate the thermal degradation of peripheral materials and elements. Accordingly, it is demanded to further enhance the heat conductivity of heat sink materials therefor (subject 5).
An object of the first invention is to provide a heat-conductive sheet which exhibits a high heat conductivity in the direction of the thickness of the sheet and which is excellent in heat resistance, durability, mechanical strength and adherence to a heating element. Other objects are to provide a process for producing the heat-conductive sheet and a composition suitable for providing the sheet.
An object of the second invention is to provide a semi-cured heat-conductive sheet which exhibits a high heat conductivity in the direction of the thickness of the sheet, which is excellent in not only adherence but also elasticity and durability, and which can be readily molded within a short period of time. Other objects are to provide a process for producing the semi-cured heat-conductive sheet, a sheet joining method and a composition suitable for providing the sheet.
An object of the third invention is to provide a heat-conductive sheet which exhibits a high heat conductivity in the direction of the thickness of the sheet, which is excellent in not only sheet surface stickiness, whilst the sheet can readily be detached in the event of any defect in, for example, the heating element, but also elasticity and durability, and which can be readily shaped within a short period of time. Other objects are to provide a process for producing the heat-conductive sheet, a sheet joining method and a composition suitable for providing the sheet.
An object of the fourth invention is to provide a heat-conductive sheet which exhibits a high heat conductivity in the direction of the thickness of the sheet and which is excellent in heat resistance, durability, mechanical strength and adherence to a heating element, while ensuring insulation from the heating element. Another object is to provide a process for producing the heat-conductive sheet.
An object of the fifth invention is to provide a heat sink structure including the above heat-conductive sheet, which heat sink structure can meet demands for high heat conductivity on, for example, electrical and electronic products.
The inventors have conducted extensive and intensive studies with a view toward solving the above problems. As a result, it has been found that the heat-conductive sheet wherein a carbon fiber and, furthermore, a magnetic substance are orientated in the cured or semi-cured binder in the direction of the thickness of the heat-conductive sheet realizes a striking enhancement of the anisotropic heat conductivity in the direction of the thickness of the heat-conductive sheet, and that the heat-conductive sheet is excellent in not only heat resistance, durability and mechanical strength but also adherence to a highly heated part. The present invention has been completed on the basis of these findings. With respect to the magnetic substance, it has been found that the magnetic substance is preferably magnetic particles and preferably adheres to the surface of the carbon fiber. (First Invention)
It has also been found that, when use is made of a curing composition for forming a heat-conductive sheet comprising a binder having a photocuring component and a thermocuring component and, contained therein, a magnetic substance and a carbon fiber, a semi-cured sheet containing the highly adherent thermocuring component in uncured form can be obtained by photocuring the photocuring component while applying a magnetic field to the composition so that the magnetic substance and the carbon fiber are orientated in the direction of the thickness of the sheet. Moreover, it has been found that, when the semi-cured heat-conductive sheet containing the uncured thermocuring component is uncured by thermocompression bonding during the process for producing semiconductor packages and the like, the thermocured heat-conductive sheet not only exhibits a high heat conductivity in the direction of the thickness but also has excellent adherence, elasticity and durability. It has also been found that the semi-cured heat-conductive sheet of the present invention can be readily produced within a short molding time. The present invention has been completed on the basis of these findings. (Second Invention)
Further, it has been found that, when use is made of a binder containing a copolymer of specified glass transition temperature and a monomer having an unsaturated bond, the obtained heat-conductive sheet not only exhibits high heat conductivity because, for example, the carbon fiber is orientated in the direction of the thickness of the sheet but also is excellent in sheet surface stickiness although being readily detachable in the event of any defect in the site of use of the heat-conductive sheet. It has also been found that the heat-conductive sheet has excellent durability and can be readily produced within a short molding time. The present invention has been completed on the basis of these findings. (Third Invention)
Still further, the inventors have found that satisfactory insulation without substantial detriment to heat conductivity can be realized by covering the above heat-conductive sheet at part or all of its surface with an electrical insulating layer. It has also been found that the obtained heat-conductive sheet is excellent in not only heat resistance, durability and mechanical strength but also adherence to a heating element. The present invention has been completed on the basis of these findings. (Fourth Invention)
Still further, the inventors have found that the above heat-conductive sheet is useful as a heat sink material in various semiconductor-related equipments, electrical equipments, electronic equipments, power generating equipments and the like. The present invention has been completed on the basis of this finding. (Fifth Invention)
The present invention is, therefore, characterized by the following.
The curing composition for forming a heat-conductive sheet according to the present invention comprises a binder, a magnetic substance and a carbon fiber.
It is preferred that the magnetic substance be a magnetic particle or adhere to the carbon fiber on its surface.
The binder preferably comprises a photocuring component and a thermocuring component. The curing composition for forming a heat-conductive sheet may further comprise a photoinitiator.
The binder may contain (A) a copolymer whose glass transition temperature is xe2x88x9230xc2x0 C. or below and (B) a monomer having an unsaturated bond.
One form of heat-conductive sheet according to the present invention is a heat-conductive sheet of given thickness comprising a binder and a carbon fiber, wherein the carbon fiber is orientated in the binder in the direction of the thickness of the heat-conductive sheet.
Another form of heat-conductive sheet according to the present invention is a heat-conductive sheet of given thickness comprising a binder, a magnetic substance and a carbon fiber, wherein the magnetic substance and the carbon fiber are orientated in the binder in the direction of the thickness of the heat-conductive sheet.
It is preferred that the magnetic substance be a magnetic particle or adhere to the carbon fiber on its surface.
The binder preferably comprises a thermocuring component and a cured component resulting from curing of a photocuring component.
It is preferred that the binder comprise (A) a copolymer whose glass transition temperature is xe2x88x9230xc2x0 C. or below and (B) a component resulting from curing of a monomer having an unsaturated bond and that the heat-conductive sheet have a sticky surface.
The heat-conductive sheet of the present invention may comprise the above heat-conductive sheet, as a heat-conductive layer, and an electrical insulating layer laminated on the heat-conductive sheet at part or all of its surface. The electrical insulating layer preferably contains a heat-conductive filler.
One mode of process for producing a heat-conductive sheet according to the present invention comprises the steps of:
forming a sheeted composition from a curing composition for forming a heat-conductive sheet comprising a binder, a magnetic substance and a carbon fiber; and
curing or semi-curing the sheeted composition while applying a magnetic field to the sheeted composition in its thickness direction so that the magnetic substance and the carbon fiber are orientated in the direction of the thickness of the sheeted composition. It is preferred that the magnetic substance be a magnetic particle form or adhere to the carbon fiber on its surface.
Another form of process for producing a heat-conductive sheet according to the present invention comprises the steps of:
forming a sheeted composition from a curing composition for forming a heat-conductive sheet comprising a binder, a magnetic substance and a carbon fiber, the binder containing a photocuring component and a thermocuring component; and
photocuring the photocuring component of the sheeted curing composition while applying a magnetic field to the sheeted composition in its thickness direction so that the magnetic substance and the carbon fiber are orientated in the direction of the thickness of the sheeted composition, thereby obtaining a semi-cured heat-conductive sheet.
A further mode of process for producing a heat-conductive sheet according to the present invention comprises the steps of:
forming a sheeted composition from a curing composition for forming a heat-conductive sheet comprising a binder, a magnetic substance and a carbon fiber, the binder containing (A) a copolymer whose glass transition temperature is xe2x88x9230xc2x0 C. or below and (B) a monomer having an unsaturated bond; and
curing by light irradiation or heating the monomer having an unsaturated bond (B) of the sheeted composition while applying a magnetic field to the sheeted composition in its thickness direction so that the magnetic substance and the carbon fiber are oriented in the direction of the thickness of the sheeted composition, thereby obtaining a sheet having a sticky surface.
The method of bonding a highly heated part and a heat sink part to each other according to the present invention comprises the steps of:
interposing between a highly heated part and a heat sink part a semi-cured heat-conductive sheet comprising a binder, a magnetic substance and a carbon fiber, the binder containing a thermocuring component and a cured component resulting from curing of a photocuring component, the magnetic substance and the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet; and
curing by thermocompression bonding the thermocuring component of the semi-cured heat-conductive sheet so that the highly heated part and the heat sink part are bonded together by means of the thus obtained heat-conductive sheet.
The method of adhering a highly heated part and a heat sink part to each other according to the present invention comprises the steps of:
interposing between a highly heated part and a heat sink part a heat-conductive sheet comprising a binder, a magnetic substance and a carbon fiber, (A) the binder containing a copolymer whose glass transition temperature is xe2x88x9230xc2x0 C. or below and (B) a cured component resulting from curing of a monomer having an unsaturated bond, the magnetic substance and the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet, the heat-conductive sheet having a sticky surface; and
sticking the highly heated part and the heat sink part to each other by means of the heat-conductive sheet.
One form of heat sink structure of the present invention comprises a highly heated part and a heat sink part joined together by means of a heat-conductive sheet of given thickness, the heat-conductive sheet containing a binder and a carbon fiber, the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet.
Another form of heat sink structure of the present invention comprises a highly heated part and a heat sink part joined together by means of a heat-conductive sheet of given thickness, the heat-conductive sheet containing a binder, a magnetic substance and a carbon fiber, the magnetic substance and the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet.
A further form of heat sink structure of the present invention comprises a highly heated part and a heat sink part joined together by means of a heat-conductive sheet of given thickness, the heat-conductive sheet comprising:
(a) a heat-conductive layer containing a binder and a carbon fiber, the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet; and
(b) an electrical insulating are laminated on the heat-conductive layer at part or all of its surface.
Still a further form of heat sink structure of the present invention comprises a highly heated part and a heat sink part joined together by means of a heat-conductive sheet of given thickness, the heat-conductive sheet comprising:
(a) a heat-conductive layer containing a binder, a magnetic substance and a carbon fiber, the magnetic substance and the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet; and
(b) an electrical insulating layer laminated on the heat-conductive layer at part or all of its surface.
It is preferred that the highly heated part be a semiconductor element, a semiconductor package, a power transistor, a PTC element, a thyristor, a heating coil of high-voltage rotating machine, a coil of voice coil, a plasma display, an. EL panel, an LD, or an LED.
According to the present invention, there is provided still a further form of heat sink structure comprising a highly heated part having on its surface a heat-conductive sheet of given thickness, the heat-conductive sheet containing a binder and a carbon fiber, the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet.
According to the present invention, there is provided still a further form of heat sink structure comprising a highly heated part having on its surface a heat-conductive sheet of given thickness, the heat-conductive sheet containing a binder, a magnetic substance and a carbon fiber, the magnetic substance and the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet.
According to the present invention, there is provided still a further form of heat sink structure comprising a highly heated part having on its surface a heat-conductive sheet of given thickness, the heat-conductive sheet comprising:
(a) a heat-conductive layer containing a binder and a carbon fiber, the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet; and
(b) an electrical insulating layer laminated on the heat-conductive layer at part or all of its surface.
According to the present invention, there is provided still a further form of heat sink structure comprising a highly heated part having on its surface a heat-conductive sheet of given thickness, the heat-conductive sheet comprising:
(a) a heat-conductive layer containing a binder, a magnetic substance and a carbon fiber, the magnetic substance-and the carbon fiber orientated in the binder in the direction of the thickness of the heat-conductive sheet; and
(b) an electrical insulating layer laminated on the heat-conductive layer at part or all of its surface.
It is preferred that the highly heated part be a semiconductor element, a semiconductor package, a power transistor, a PTC element, a thyristor, a printed board, a heater of image forming device, a high-temperature fluid, or a light emitter such as an electric lamp.